Android: Fix a bug at tab switching

TabWebContentsDelegateAndroid should always be able to get
the fullscreen manager to return the right control height
(https://crrev.com/c/1475677).

This CL replaces FullscreenManager.from() which can return
|null| while tabs are in transition with the reinstated
getFullscreenManager() that always returns the ChromeActivity's
FullscreenManager correctly.

Bug: 963820
Change-Id: Iafef11ed7dff14de38a305e6494eb0f3f83b6b46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615406Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661237}

chrome/android/java/src/org/chromium/chrome/browser/tab/TabWebContentsDelegateAndroid.java

@@ -473,21 +473,28 @@ public class TabWebContentsDelegateAndroid extends WebContentsDelegateAndroid {
         mTab.getActivity().setOverlayMode(useOverlayMode);
     }
 
+    private ChromeFullscreenManager getFullscreenManager() {
+        // Following get* methods use this method instead of |FullscreenManager.from()|
+        // because the latter can return null if invoked while the tab is in detached state.
+        ChromeActivity activity = mTab.getActivity();
+        return activity != null ? activity.getFullscreenManager() : null;
+    }
+
     @Override
     public int getTopControlsHeight() {
-        FullscreenManager manager = FullscreenManager.from(mTab);
+        FullscreenManager manager = getFullscreenManager();
         return manager != null ? manager.getTopControlsHeight() : 0;
     }
 
     @Override
     public int getBottomControlsHeight() {
-        FullscreenManager manager = FullscreenManager.from(mTab);
+        FullscreenManager manager = getFullscreenManager();
         return manager != null ? manager.getBottomControlsHeight() : 0;
     }
 
     @Override
     public boolean controlsResizeView() {
-        FullscreenManager manager = FullscreenManager.from(mTab);
+        FullscreenManager manager = getFullscreenManager();
         return manager != null ? ((ChromeFullscreenManager) manager).controlsResizeView() : false;
     }